I. Field of the Invention
The present invention relates to a method of determining the degradation of low alloy steel and, more particularly, to a method of nondestructively measuring the degree of degradation of low alloy steel, embrittled in a high temperature atmosphere over a long period of use, using an electrochemical measurement.
II. Description of the Prior Art
A metallic material, such as steel, is given various heat treatments to obtain the mechanical property required for its application, and is then used. However, in a material which is used at a comparatively high temperature of several hundreds degrees centigrade, the mechanical property is degraded. Although such degradation in the mechanical property is often due to creep deformation caused by a regular stress, it is sometimes based on various textural changes.
For example, in a rotor of a steam turbine which operates at a high temperature, phosphorus as an impurity in steel segregates at crystal grain boundaries, thereby weakening the grain boundary strength. As a result, the toughness of the steel is significantly lowered, i.e., temper embrittlement occurs. Observation through a microscope or using a Charpy impact test is the most direct method of determining the degree of degradation of such a metallic material. These methods are accurate determining methods, but they have a big disadvantage in that extracting a test specimen from the metallic member is required.
In view of the foregoing, a nondestructive test method of electrochemically detecting a textural change in the metallic material has recently been developed. In the above conventional methods, a metallic material to be tested is used as a working electrode to be brought into contact with an appropriate electrolytic solution. A reference electrode and auxiliary electrode are dipped into the same electrolytic solution, and a polarization property is measured by these electrodes, thereby detecting a textural change in the metallic material by an appropriate parameter obtained by this measurement or by comparison of the obtained parameter with that of a virgin metallic material. In this case, as a parameter, an anodic or cathodic peak current density of a polarization curve, a critical pitting potential, a repassivation potential, a passivation current density, a charge quantity flowing in a potential range of interest, and a ratio of parameters during normal potential sweep and reverse sweep may be used.
These measurements, however, of polarization behavior generally have poor reproducibility. Though conventional methods have been improved to compensate for this defect, because of the large scattering in measured values, it is difficult to obtain a clear relationship between the measured values and the degree of embrittlement of a metallic material. In addition, when an electrochemical measurement of the degree of embrittlement of a metallic material is performed, the selection of an electrolytic solution is an important problem. Nevertheless, sufficient studies and developments have not been made concerning such solutions, and a method of determining the brittleness of a metallic material with high accuracy has not yet been provided.